The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In general, a CVVT system continuously changes opening/closing timings by changing the phase of a camshaft in accordance with the revolution per minute (RPM) of an engine and the load on a vehicle. In general, the CVVT systems largely include a crank angle sensor that senses a rotational angle of a crankshaft, a cam angle sensor that senses a rotational angle of a camshaft, a variable valve timing unit that is connected to a side of a camshaft through a timing belt and advances or retards the camshaft, and an ECU that controls an OCV (Oil Control Valve) so that oil is supplied to an advancing chamber or a retarding chamber of the variable valve timing unit in response to signals from the crank angle sensor and the cam angle sensor.
The variable valve timing unit comprises a stator connected by a timing belt to receive torque from the crankshaft and a vane-shaped rotor integrally combined with the camshaft and rotating relative to the stator. A chamber divided into the advancing chamber and the retarding chamber by the rotor is formed in the stator, so when oil is supplied into the advancing chamber through the OCV, a phase difference is generated between the rotor and the stator and the camshaft rotates, and thus, the timing of a valve changes. On the contrary, when oil is supplied into the retarding chamber through the OCV, a phase difference is generated between the rotor and the stator in the opposite direction to that when oil is supplied into the advancing chamber, so the timing of the valve is retarded.
Further, a lock pin is formed on the rotor to fix the rotor to the stator when an engine stops, and a locking hole for locking the lock pin is formed in the stator. The ECU adjusts valve timing for the cam in accordance with a crank position in response to signals from the crank angle sensor and the cam angle sensor. When the OCV allows the cam to rotate in response to a control signal from the ECU, the cam angle sensor detects the position of the camshaft and feeds it back to the ECU. The ECU estimates the amount of rotation of the cam on the basis of the fed-back position information of the camshaft and transmits a signal for controlling the position of the camshaft back to the OCV on the basis of the estimated amount of rotation of the cam. The CVVT systems are controlled by this control logic.
On the other hand, in order to smoothly perform the feedback function, a control logic for the OCV according to the crank position and the cam position is mapped in the ECU, and when the mapped position of the camshaft and the cam position detected by the cam angle sensor are different, the ECU controls an oil control valve, so the rotation of the camshaft is increased/decreased.
In an intermediate phase of CVVT, a lock pin on the rotor is locked into a lock pin hole between the advancing chamber and the retarding chamber while the RPM of an engine is reduced, thereby preparing for later engine start. The action that the lock pin is automatically locked into the lock pin hole while the RPM of an engine reduces is called ‘self-lock’.
The self-lock is a function that allows CVVT can mechanically return to an accurate position without specific adjustment so that operational stability of an engine can be maintained in other periods except for the operation period of the CVVT, that is, when the engine is idling or started.
However, when the valve timing reaches the most retarded position, not returning to the intermediate phase and an engine of a vehicle is idling, a surge tank is not vacuumized and the internal pressure of the surge tank increases up to the atmospheric pressure, so the performance of a brake using the vacuum of the surge tank is deteriorated.
Further, when the valve timing reaches the most retarded position, not returning to the intermediate phase, excessive overlap of valve timing is generated between an intake valve and an exhaust valve, so the operational stability of the engine decreases and vibration of the engine increases, and in some cases, the engine stops.
In particular, when an Atkinson cycle is applied to a vehicle, it is desired to retard the timing of closing the intake valve, using a CVVT, in order to maximize the effect of the Atkinson cycle, but an intake CVVT of the related art is fixed to the most retarded position for starting and idling, so compressive pressure is insufficient and the engine is not normally started. Accordingly, when such an engine is started or is idling, it is possible to set the timing of an intake valve to a common MPI (Multi-Position Injection) level, and the engine may have a system that retards the timing of the intake valve in a certain period where fuel efficiency can be improved.
That is, an intermediate phase CVVT system has a basic position at an intermediate position and retards intake valve timing while a vehicle is driven, so an intermediate phase CVVT of the related art can be applied only to a V-6 type or horizontal engine in the way of controlling an intermediate phase using cam torque.